A nominal mix is concrete proportioned by fixed, pre-set volumetric ratios of cement : sand : aggregate — such as 1:2:4 for M15, 1:1.5:3 for M20 and 1:1:2 for M25. These ratios are conventional rules of thumb that give roughly the intended strength without laboratory testing, which makes them convenient for small jobs and routine residential work.
Nominal mixes are deliberately conservative: they tend to use a little more cement than strictly necessary to guarantee strength despite variations in materials and site conditions. IS 456 permits nominal mixes only up to about M20; for M25 and above it requires a design mix, where the proportions are calculated and verified by trials for the specific materials.
So while you will often see 1:1:2 quoted for M25, that is an approximation, not a code-sanctioned nominal ratio. For most house construction, nominal mixes are perfectly adequate; for higher grades or large structural work, a proper design mix is both safer and more economical.